The Faint Young Sun Paradox

&The

Geochemical C Cycle

& Climate on Geologic

Time Scales

12.842 Paleo Lecture #4Fall 2006

The ‘Faint Young Sun Paradox’

Faint Young Sun

Paradox

4 1H-->4HeIncr.

density= incr.

luminosityLiquid H2O existed >3.5 Ga (sed. rocks, life, zircon 18O)

Faint young sun paradox

Contemporary Solar Variability

•Contemporary Solar Variability ~0.1%

•Associated with 11-year sunspot cycle

Greenhouse Gases absorb

IR radiatio

n efficien

tly

(1)Molecules acquire energy when they absorb photons.

(2)This energy will be released later as re-emitted photons.

(3)Atmospheric molecules are rotating rapidly (and in aggregate, randomly), so the re-emitted energy is random in direction.

(4)So: half of the re-emitted radiation is directed back towards the earth.

Earth’s Energy

Balance

Simple Planetary Energy Balance •Likely

solution to FYSP requires understanding of Earth’s energy

balance (& C cycle)

Adapted from Kump et al.

(1999)

€

σ =2πk4

h3c2

π 4

15= 5.67 •10−8 w m−2 °K−4

Energy Absorbe

d

Adapted from Kump et al.

(1999)

Neither albedo nor geothermal heat flux changes can keep the earth from freezing w/ 30% lower S

Adapted from Kump et al.

(1999)

Lower S compensated by larger greenhouse effect?

Adapted from Kump et al.

(1999)

But: the atmosphere is a leaky greenhouse…

If we assume that the atmospheric gas composition is what it is today, and then do the full radiative calculations assuming that the atmosphere does not convect, then the earth would be ~30°C warmer than it is now. That is, the earth’s greenhouse effect is only ~50% efficient.

The difference is due to convection: when the near-surface air warms up, it rises in the atmosphere and can lose radiation to space more effectively.

Precambrian Precambrian ppCOCO22 EstimatesEstimates

Kaufman & Xiao (2003), Nature Vol. 425: 279-282.

Earth’s Climate History:

Mostly sunny Mostly sunny with a 10% with a 10% chance of chance of

snowsnow

•What caused these climate

perturbations?

Carbon Cycle:

Strong driver of

climate on

geologic

timescales

Biosphere, Oceans and Atmosphere 3.7x1018 moles

Crust

Corg1100 x 1018

mole

Carbonate5200 x 1018

mole

Mantle

100,000 x 1018 mole

Earth's Carbon Budget

Species Amount(in units of 1018 gC)

Residence Time (yr)* 13 C

o PDB**

Sedimentary carbonate-C 62400 342000000 0

Sedimentary organic-C 15600 342000000 -24

Oceanic inorganic-C 42 385 +0.46

Necrotic-C 4.0 20-40 -27

Atmospheric-CO2 0.72 4 -7.5

Living terrestrial biomass 0.56 16 -27

Living marine biomass 0.007 0.1 -22

Carbon Reservoirs, Fluxes and Residence TimesCarbon Reservoirs, Fluxes and Residence Times

Rapid TurnoverRapid Turnover

Steady State & Residence Time

The Residence time of a molecule is the average amount of time it is expected to remain in a given reservoir.

Atmospheric CO2

760 Gton C

Inflow:60 Gton C/yr

Outflow:60 Gton C/yr

PhotosynthesisRespiration

Steady State: Inflows = OutflowsAny imbalance in I or O leads to changes in reservoir size

1 Gton = 109 * 1000 kg = 1015 g

Example: tR of atmospheric CO2 = 760/60 = 13 yr

The bio-geochemical carbon cycle

Chemical Weathering = chemical attack of rocks by dilute acid

CO2 + H2O <---> “H2CO3”

----------------------------------------------

1. Carbonate Weathering:

CaCO3 + “H2CO3” --> Ca2+ + 2 HCO3-

2. Silicate Weathering: CaSiO3 + 2 “H2CO3” --> Ca2+ + 2HCO3

- + SiO2 + H2O

============================= • Carbonates we ather faster than silicates

biogeochemical carbon cycle #2

Carbonate

rocks weather faster than

silicate

rocks…

Products of

weathering precipitat

ed as CaCO3 & SiO2 in ocean

Kump et al. (1999)

CaCO3 weathering is cyclic (CO2 is not lost from the system), but calcium silicate weathering results in the loss of CO2 to solid CaCO3:

CaCO3 weathering cycleCaCO3 weathering:

CaCO3+ CO2 + H2O => Ca2+ + 2 HCO3-

CaCO3 sedimentation:

Ca2+ + 2 HCO3- => CaCO3 + CO2 + H2O

Silicate weathering cycle(?)Silicate weathering:

CaSiO3 + 2 CO2 + 2H2O => Ca2+ + Si(OH)4 + 2 HCO3-

CaCO3 and SiO2 sedimentation:

Ca2+ + 2 HCO3- + Si(OH)4 => CaCO3+ SiO2•2H2O + 1

CO2 + H2O

The weathering of other aluminosilicates results in the loss of CO2

AND makes the ocean saltier and more alkaline:

Potassium feldspar weathering “cycle”weathering:

2 KAl2Si2O8 + 2 CO2 + 2H2O => 2 K+ + 2 HCO3- + 2 Si(OH)4 +

Al2O3

(solid)sedimentation:

2 K+ + 2 HCO3- + 2 Si(OH)4 + Al2O3 => 2 SiO2•2H2O + Al2O3 + 2

K+ + 2 HCO3-

Problem:

As CO2 is buried as CaCO3 in sediments, why doesn’t CO2 eventually vanish from the

atmosphere?

(It would only take ~400,000 years of silicate weathering to consume all of the carbon in today’s ocean/atmosphere

system)

Net reaction

of geochemical carbon cycle (Urey

Reaction)

Net Reaction of Rock Weathering

+

Carbonate and Silica Precipitation in Ocean

CaSiO3 + CO2 --> CaCO3 + SiO2

• CO2 consumed • Would deplete atmospheric CO2 in 400

kyr • Plate tectonics returns CO2 via

Metamorphism and Volcanism

-------------------------------------

Carbonate Metamorphism

CaCO3 + SiO2 --> CaSiO3 + CO2 • CO2 produced from subducted marine

sediments

Carbonate-Silicate

Geochemical Cycle

Stanley (1999)

•CO2 released from volcanism dissolves in H2O, dissolves rocks•Weathering products transported to ocean by rivers•CaCO3 precipitation in shallow & deep water•Cycle closed when CaCO3 metamorphosed in subduction zone or during orogeny.

How are CO2

levels kept in balance?

Feedbacks…

• Geologic record indicates climate has rarely reached or maintained extreme Greenhouse or Icehouse conditions....

• Negative feedbacks between climate and

Geochemical Carbon Cycle must exist • Thus far, only identified for Carbonate-

Silicate Geochemical Cycle:

Temp., rainfall enhance weathering rates (Walker et al, 1981)

(i.e., no obvious climate dependence of tectonics or organic carbon geochemical cycle.)

Adapted from Kump et al.

(1999)

The Walker (1981) feedback for CO2

regulation • (1) CO2 emitted by volcanoes• (2) CO2 consumed by weathering• (3) If (1) is greater than (2), CO2 levels in the atmosphere increase.

• (4) As atm. CO2 rises, the climate gets (a) warmer and (b) wetter (more rainfall).

• (5) Warmer and wetter earth weathers rocks faster. CO2 is removed from the atmosphere faster.

• (6) CO2 levels rise until the weathering rate balances volcanic emissions. Steady-state attained (until volcanic CO2 emissions rise or fall).

The (modified) BLAG [Berner, Lasaga and Garrels) mechanism for long-term CO2 regulation

• Walker mechanism plus consideration of changes in sea floor spreading rate (induces ~100 myr lag time between CO2 emissions and ultimate recycling via Urey reaction), volcanism, and other factors influencing carbon cycle (organic deposition and weathering).

• Modified by J. Edmond to include irregularity of CaCO3 deposition (shallow sediments and some basins get “more of their fair share” of CaCO3 deposition, and spreading and subduction are not closely linked spatially (e.g. Atlantic

spreads but has little subduction).